Cathode ray tube (CRT) displays, such as those commonly used in desk-top computer screens, function as a result of a scanning electron beam from an electron gun impinging on phosphors on a relatively distant screen. The electrons increase the energy level of the phosphors. When the phosphors return to their normal energy level, they release photons which are transmitted through the glass screen of the display to the viewer.
Flat panel displays seek to combine the cathodoluminescent-phosphor technology of CRTs with integrated circuit technology to create thin, high resolution displays wherein each pixel is activated by its own electron emitter. This type of display technology is becoming increasingly important in appliances requiring lightweight portable screens.
It is important in flat panel displays of the field emission cathode type that an evacuated cavity be maintained between the cathode electron emitting surface and its corresponding anode display face (also referred to as an anode, cathodoluminescent screen, display screen, faceplate, or display electrode).
There is a relatively high voltage differential (e.g., generally above 200 volts) between the cathode emitting surface (also referred to as base electrode, baseplate, emitter surface, cathode surface) and the display screen. It is important that electrical breakdown between the electron emitting surface and the anode display face be prevented. At the same time, the narrow spacing between the plates is necessary to maintain the desired structural thinness and to obtain high image resolution. The spacing also has to be uniform for consistent image resolution, and brightness, as well as to avoid display distortion, etc. Uneven spacing is much more likely to occur in a field emission cathode, matrix addressed flat vacuum type display than in some other display types because of the high pressure differential that exists between external atmospheric pressure and the pressure within the evacuated chamber between the baseplate and the faceplate. The pressure in the evacuated chamber is typically less than 10.sup.-6 torr.
Small area displays (e.g., those which are approximately 1" diagonal) do not require spacers, since glass having a thickness of approximately 0.040" can support the atmospheric load, but as the display area increases, spacer supports become more important. For example, a screen having a 30" diagonal measurement will have several tonnes of atmospheric force exerted upon it. As a result of this tremendous pressure, spacers will play an essential role in the structure of the large area, light weight, displays.
Spacers are incorporated between the display faceplate and the baseplate upon which the emitter tips are fabricated. The spacers, in conjunction with thin, lightweight, substrates support the atmospheric pressure, allowing the display area to be increased with little or no increase in substrate thickness.
Spacer structures must conform to certain parameters. The supports must 1) be sufficiently non-conductive to prevent electrical breakdown between the cathode array and the anode, in spite of the relatively close interelectrode spacing (which may be on the order of 100 microns), and relatively high interelectrode voltage differential (which may be on the order of 200 or more volts); 2) exhibit mechanical strength such that they exhibit only slow deformation over time to provide the flat panel display with an appreciable useful life; 3) exhibit stability under electron bombardment, since electrons will be generated at each of the pixels; 4) be capable of withstanding "bakeout" temperatures of around 400.degree. C. that are required to create the high vacuum between the faceplate and backplate of the display; and 5) be of small enough size so as to not to visibly interfere with display operation.
Various types of spacers have been developed. A few examples are disclosed in U.S. Pat. Nos. 4,183,125, entitled "Gas Panel Spacer Technology," 4,091,305, entitled "Method of Making an Insulator-support for Luminescent Display Panels and the Like," 4,422,731, entitled "Display Unit with Half-stud, Spacer, Connection Layer and Method of Manufacturing," and 4,451,759, entitled "Flat Viewing Screen with Spacers between Support Plates and Method of Producing Same".
U.S. Pat. No. 4,923,421 entitled, "Method for Providing Polyimide Spacers in a Field Emission Panel Display," discloses the use of spacer supports in field emission displays. In the above mentioned patent, Brodie et al. describe a process wherein spacers are formed by applying a layer of material to one of the plate surfaces, patterning the material, and then removing the material except for the portions which form the spacers.
There are several drawbacks to the spacers and methods described in the above cited patents. One disadvantage is need for the spacer supports to be relatively large, having diameters in the range of 50 microns, in order to render innocuous the small amount of isotropic distortion that inevitably occurs during anisotropic (plasma) etches. In other words, if the spacers are too narrow, they will tend to curve due to undercutting during the etching process which is used to eliminate the material surrounding the spacer.
Those known processes which involve the use of attaching and aligning pre-made spacers to the electrodes are very unreliable, tedious and expensive.